The present invention relates to the magnetic resonance imaging arts. It finds particular application in conjunction with medical magnetic resonance imaging systems and will be described with particular reference thereto. It is to be appreciated, however, that the present invention may also find application in conjunction with other types of magnetic resonance imaging systems, magnetic resonance spectroscopy systems, and the like.
In magnetic resonance imaging, a substantially uniform main magnetic field is generated within an examination region. The main magnetic field polarizes the nuclear spin system of a subject being imaged within the examination region. Magnetic resonance is excited in dipoles which align with the main magnetic field by transmitting radio frequency excitation signals into the examination region. Specifically, radio frequency pulses transmitted via a radio frequency coil assembly tip the dipoles out of alignment with the main magnetic field and cause a macroscopic magnetic moment vector to precess around an axis parallel to the main magnetic field. The precessing magnetic moment, in turn, generates a corresponding radio frequency magnetic signal as it relaxes and returns to its former state of alignment with the main magnetic field. The radio frequency magnetic resonance signal is received by the radio frequency coil assembly, and from the received signals, an image representation is reconstructed for display on a human viewable display.
In certain medical MRI applications, it is advantageous to perform imaging scans over a limited field of view and depth of penetration of specific regions of the patient being examined. Such regions may include the anus, the prostate, the cervix, and other regions associated with internal cavities of a patient. RF receive coils of the intracavitary or endocavitary type are generally used to image these regions as the proximity of the coils in such applications provides improved signal-to-noise ratio over a limited field of view and depth of penetration.
Previously, intracavitary or endocavitary RF receive coils made use of an active RF coil element contained within an inflatable non-permeable balloon. An electrical cable interfaced the active RF coil to external electrical circuitry that was used to interface the coil with the magnetic resonance imaging system. The device would be inserted into a cavity associated with the region of interest, for example a patients rectum, and the balloon would then be inflated. Finally, the external electrical interface would be used to tune and match the coil to the MRI system. Typically, such endocavitary coils were disposable and would not be reused for multiple scans. Additionally, the active RF coil element was not rigidly formed, and as such, each individual probe had to be tuned and matched.
Another previous form of an endocavitary probe was reusable for a limited number of times before disposal. This form employed a loose latex sheathing or over cover which was disposed after each patient. However, the device was reused for only a limited number of times.
Other types of reusable coils are constructed of medical grade plastic. Such coils are subject to surface disinfection after each use. Coils which are encased in the plastic are typically immersed in a liquid disinfectant or sterilant, such as alcohol, hydrogen peroxide, or the like after each use.
Because these coils come in contact with mucus membranes and blood barriers, high level disinfection or sterilization are desirable. Heat sterilization tends to degrade plastic parts and may harm on board electronics. Balloons are particularly sensitive to heat degradation. The coils are often constructed of multiple pieces of plastic, and thus has seams where the pieces meet. Microscopic organisms can become lodged in these crevices where liquid sterilants cannot assuredly eliminate them.
The present invention contemplates a new and improved endocavity RF coil assembly for an MRI apparatus which overcomes the above referenced disadvantages and others.
In accordance with one aspect of the present invention, an endocavity RF coil assembly for an MRI apparatus is provided. The endocavity RF coil assembly includes a reusable probe. The reusable probe includes a hollow outer housing having a closed distal end and an open proximate end. The distal end is formed to fit into a cavity of a subject being examined. An active RF coil element is rigidly formed about an internal sleeve which is located within the distal end of the outer cover. A tuning and matching circuit disposed within the outer cover on the proximate end side of the active RF coil element is arranged on a printed circuit board and attached to the active RF coil element. An over-molded form is connected to the proximate end of the outer cover. The over-molded form is arranged such that it seals the proximate end of the outer cover closed. The probe includes an epoxy coating.
In accordance with a more limited aspect of the present invention, the endocavity RF coil assembly includes a non-magnetic magnetic RF cable connected to the tuning and matching circuit. The non-magnetic RF cable extends out of the open proximate end of the outer cover through the over-molded form to a preamplifier of the MRI apparatus. The RF cable has an outer cable insulation and a an MR compatible and biocompatible outer jacket extruded over the outer cable insulation.
In accordance with a more limited aspect of the present invention, the over-molded form and the outer jacket of the RF cable are both made out of a PVC plastic which forms a seal therebetween.
In accordance with a more limited aspect of the present invention, the internal sleeve and the outer housing are made form a medical grade ABS plastic.
In accordance with a more limited aspect of the present invention, the endocavity RF coil assembly includes a plurality of grooves cut into the outer surface of the outer housing at a proximate end. The grooves are arranged to completely encircle the outer cover such that a portion of the over-molded form is embedded therein sealing the connection between the over-molded form and the outer housing.
In accordance with a more limited aspect of the present invention, some of the plurality of grooves are arranged with flat surfaces such that the over-molded form embedded therein prevents the over-molded form from rotating with respect to the outer housing.
In accordance with a more limited aspect of the present invention, the over-molded form is a polyhedron having a plurality of sides. The plurality of sides are each indexed with a representative indicia that indicates the orientation of the active RF element within the subject being examined.
In accordance with a more limited aspect of the present invention, the over-molded form is in the shape of a sphere.
In accordance with a more limited aspect of the present invention, the probe includes material which emits a limited magnetic resonance signal which is used to track the position of the probe relative to the subject being examined.
In accordance with a more limited aspect of the present invention, the printed circuit board is positioned and electronic components are located thereon such that image anomalies caused thereby are eliminated.
One advantage of the present invention is that the probe is reusable.
Another advantage of the present invention is that it can be manufactured to meet the electrical and mechanical required compliance standards such as UL 544 and IEC 601.
Another advantage of the present invention is that external circuitry for performing tuning and matching adjustments on a per-patient basis is eliminated.
Another advantage of the present invention is the prevention of the ingress of fluids into the interior of the reusable probe.
Another advantage of the present invention is the protection of subjects from RF burn.
Yet another advantage resides in the ease of sterilization.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.